In recent years, a laser crystallization technique for forming a semiconductor film having a crystalline structure (hereinafter, a crystalline semiconductor film) by irradiating an amorphous semiconductor film which is formed over a glass substrate with a laser beam has been researched well. A crystalline semiconductor film is used because of high mobility as compared with an amorphous semiconductor film. Therefore, a TFT is formed using this crystalline semiconductor film, and such a TFT has been used for an active matrix liquid crystal display device, an organic EL display device, or the like where a TFT for a pixel portion or TFTs for a pixel portion and a driver circuit is/are formed over a sheet of a glass substrate.
As a crystallization method, a thermal annealing method using an annealing furnace, a rapid thermal annealing method (RTA method), a laser annealing method (a crystallization method by laser irradiation), or the like can be given. In a case of using a solid phase growth method like a thermal annealing method, high-temperature processing at greater than or equal to 600° C. is performed; therefore, an expensive quartz substrate that can withstand the high temperature is needed, which increases a manufacturing cost. On the other hand, when a laser is used for crystallization, crystallization can be performed by making only a semiconductor film absorb heat without increasing a temperature of a substrate too much. Therefore, a substance having a low melting point such as glass or plastic can be used for a substrate. Accordingly, a glass substrate which is inexpensive and easily processed in a large-sized area can be used; thus, production efficiency has been significantly improved.
There is a crystallization method by an excimer laser, which is a pulse laser, as one of the laser crystallization methods. A wavelength of an excimer laser belongs to an ultraviolet region, and absorptance with respect to silicon is high. Therefore, silicon is selectively heated when an excimer laser is used. For example, in a case of using an excimer laser, a laser beam having a rectangular shape of approximately 10×30 mm that is emitted from a laser oscillator is shaped by an optical system into a linear beam spot of several hundreds of μm in width and greater than or equal to 300 mm in length and silicon over a substrate is irradiated with the laser beam. Here, a rectangular shape with a high aspect ratio, or an elliptical shape is referred to as a linear shape. By irradiating silicon over a substrate with a linearly processed beam spot while relatively scanning the beam spot, annealing is performed and a crystalline silicon film is obtained. High productivity can be obtained by scanning the beam spot in an orthogonal direction to a direction of a length (a major axis) of the beam spot.
As another laser crystallization method, there are crystallization methods by a continuous-wave laser (hereinafter, referred to as a CW laser) and a pulse laser having a repetition rate as high as greater than or equal to 10 MHz. These lasers are formed into linear beam spots; thus, a crystalline silicon film is obtained by irradiating a semiconductor film with this beam spot while scanning the beam spot. By using this method, as compared with a crystal that is obtained by irradiation of an excimer laser beam, a crystalline silicon film having a region of an extremely large grain size (hereinafter, referred to as a large grain crystal) can be formed (for example, see Reference 1: Japanese Published Patent Application No. 2005-191546). When this large grain crystal is used for a channel region of a TFT, few crystal grain boundaries are included in a channel direction; therefore, an electric barrier against a carrier such as an electron or a hole gets lower. Consequently, it is possible to manufacture a TFT having mobility of approximately 100 cm2/Vs.